Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat. No. 4,769,292] demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly consists of a host material doped with a guest material. Wherein the layer structures are denoted as HTL/LEL/ETL. Further, there are other multilayer OLEDs that contain more functional layers in the devices. At the same time, many kinds of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
As is known, most EL materials are sensitive to oxygen and/or moisture and/or other components. For example, tris(8-hydoxyquinoline) aluminum (Alq) is known to react with water [F. Papadimitrakopoulos et al, “A Chemical Failure Mechanism for Aluminum(III) 8-Hydorxyquinoline Light-Emitting Devices”, Chem. Mater. 8, 1363 (1996)]. The use of a vacuum or low oxygen and/or water conditions during device fabrication steps can help reduce the failure rate of OLED devices. However, inadvertent contamination of the device by oxygen, moisture, and/or other components is possible during or between deposition steps or anytime there is an apparatus transfer or delay between steps. This can lead to inferior EL performance due to luminance quenching and/or higher carrier injecting barriers caused by the contaminants.
In a full-color organic display, there is an array of colored pixels such as red, green and blue color (commonly referred to as RGB pixels). Precision patterning of the colored LEL is required to produce the RGB pixels. The RGB pixels are fabricated through several steps, and each colored LEL needs its own special patterning and evaporation steps. Only after all of the three colored LELs are pixelized, the subsequent common ETL and other common layers can be fabricated on the top of the colored pixels. Therefore, there is a certain amount of waiting time during each pixel patterning step and between the steps of the pixel patterning and the subsequent ETL fabrication. During the waiting interval, the surface of the individual colored LEL's is exposed to an environment which will have different colored molecules. Therefore, contamination is inevitable although the environment is under the vacuum conditions. As a result, the contaminated pixels could have inferior EL performance, such as sudden initial luminance drop, or worse operational stability. Moreover, in some cases, the LEL has to be exposed to atmosphere for a certain period of time before continuing the subsequent fabrication steps. In this situation, the surface of the air exposed LEL is severely contaminated by moisture and other unwanted components resulting in inferior EL performance. In addition, during the fabrication of some types of OLEDs by using polymers as the LEL (polymeric OLEDs), the spin-coated or ink-jet printed LEL has to be annealed at a specific temperature over a period of time before continuing the subsequent fabrication steps. Therefore, the annealed surface is also severely contaminated by unwanted components resulting in inferior EL performance.